Carb sync-ing

Guys I'm still pursuing with the Sync-Link.

Engine warmed up, with carbs isolated  from the Sync-Link cables, on tick-over l dial in each carb via the throttle stop screw until each carb is reading the same (Say 5 on the Snail Gauge) at tick-over. So in theory, the carbs should now be synced. At tick-over yes they are. So before l connect the cables for curiosity l place a 1-5mm washer between the throttle stop arm and the idle screw. This increases the revs to 2500rpm. Now it gets interesting, looking at the Snails ( one on each carb) one reads 18 on the left bank and the rights reads 12.

This is the big question??

Am l right in thinking they should be reading the same?????

Any input why they're not would be great.

 

Rich


Are we ever finished ;-)

Original Post

The snails should read the same.....If not, you have some adjusting to do. I have always found it better to come down to the lowest reading...ie....bring the 18 down towards the 12. The 12 will probably go up so watch closely..... There are others on here that have the same system you do so they can give better advice.   I use a Vintage Speed "PUSH" linkage system which allows slack at idle but allows sync adjustments after all the slack is taken up and set at about 2500 rpm. The Hex Bar System is junk IMHO ......Bruce

Everybody here (and over on TheSamba) loves, Loves, LOVES the hex-bar, Bruce. If they can't love it anymore, they move on to a Scat linkage, or CSP, or anything (ANYTHING!) but a sync-link. Apparently, because a sync-link costs 2x what the CB linkage does, guys feel it's over-priced. It's cheaper than buying 3 different linkages, and feeling less than satisfied.

Stan, I'm with you but I've got it it now so need to persevere.

I was using the centre pivot linkage that Raby's sold when l ordered the Dtm many years ago. I did find that good in the last engine set up. For curiosity l did shelve the Sync recently and put the Raby unit back on and noticed the Sync was much smoother even with this problem. So I'm back with the Sync-Link again. 

So putting that aside, this problem I have once the carbs are dialled in at idle without any linkage connected. As soon l move  it of idle which l used a washer to hold both carbs exactly the same from idle. I'm getting differant readings, 12 and 18. If l pull the washers the carbs return to idle and both read 5 on the Snails. If l put the washers back in place and adjust bringing the 18 down to 12, once l pull the washers and it returns back to idle they're out of sync and runs uneven.

In the past I've always sync'd them at idle and left it at that. Presuming  the'd be synced of idle also.  So can be causing this and what can l adjust to keep them sync'd both on and of idle??? 

I did think maybe its down to differant compression on both banks. So l took a reading and got these. Could it be the 5psi causing this?? 

Cylinder 1 140 psi
Cylinder 2 135 psi
Cylinder 3 135 psi
Cylinder 4 140 psi

One other thing that I've not tried as I'm writing this is the Air adjusting screw??. Would this be an option?? If not I'm stumped, and any input or direction to go in would be greatly appreciated.

 

Richard,

Sorry for last night's post. I love the sync-link, and it's the only way to fly. You've got the best possible part, so on to the question.

You've arrived at the deep end of the pool here. There's probably never been a dual carb set-up (or dual throttle-body setup, for that matter) that draws in perfect sync all the way up to WOT. There are too many variables-- how the throttle butterflies mount on the shafts, etc. Fortunately, it doesn't matter as much as you would think. What matters is what happens at idle up to about 1/4 open.

The good new is that it's pretty much impossible for a sync-link to pull the throttles in anything but a linear manner, as opposed to a hex-bar, which has so much slop as to be pretty much a guess. If I'm understanding the nature of your question, you're in perfect sync at idle, just not at a partial throttle opening.

The thing about idle is that you can (and should) be in sync without any throttle linkage at all. Adjustments to idle are made with the throttle-stop screws with the linkage out of play. Your linkage should always allow the throttles (both of them) to go back to the stops at idle. It sounds as if when you adjust the carbs to pull in sync at slight openings, the throttles don't return to the stops at idle. You can't have it work that way.

What you are looking for is very slight movement of the accelerator cable producing a linear movement on both carbs. I don't think I'd be looking at it (initially) with a snail, I'd probably be using a feeler gauge or something similar. I'd set up the linkage so that a tiny amount of movement on the accelerator cable (which will pull the 3/4 butterfly) has a corresponding pull on the 1/2 carb.

You could do this without the engine running, by holding the 3/4 throttle open with a feeler gauge between the adjustment screw and the stop, then seeing if the 1/2 carb is open the same amount (you'd check with a second feeler gauge). The only real adjustment you are going to be able to get is just off the throttle stops- making sure the throttles crack at just the same time. Once you've got it set up where they are on the stops at idle, and just cracking together, then I'd check with the snail.

Good luck.

Stan and I disagree on hex bar vs. sync link, but here that really doesn't matter. Hex bars are sloppy unless modified with heim joints. I helped Lenny set up his sync link, and he is happy. I agree, it's a nice system.

Richard, you're going about this all wrong. Throw away the washers, they aren't helping you. Ok, you have sync at idle using the throttle stops. Great.

What you need to do is pull the throttle open very slightly, and hold rpm around 1500 to 2500. Even 3000 works, which is what I used to do. A hand throttle helps if you have one, I rigged one up. Doesn't matter much what rpm, just pick an rpm and STICK TO IT. Have a friend hold it there, using the throttle pedal, because that's how it will work when you're driving it. IMHO, 1500 is best as you want it perfectly synced just off idle, this is where your foot is when you are cruising down the road at 55-65.

Use the snail and the adjuster between the carbs to bring them as close as possible. It should be possible to get them spot-on. For reference, my synchrometer reads about 16-18 at this rpm. Take it for a drive, you're done. It is impossible to get them perfect at all rpm as Stan says. Idle and just off idle matter, once the plates are 1/4 open and beyond doesn't.

Guys thanks for the feedback. 

Danny l used the washers purely to hold the revs, in this instance @ 2500rpm. I did not want to connect any linkage until they are reading the same or similar.   If l can't get them reading without the linkage then no point hooking everything else up.

I've been doing some digging on the web regarding the air bleed screws. A lot of question with uncertain answers but nothing concrete. 

Decided to call a carb importer and parts supplier here in the U.K.. after a indepth chat etc he advised not to adjust the airbleed screws to sync the 2 carbs. He did advise to swap the carbs over and see if the problem persisted on the same side or if it had swapped sides. Indicating a carb or an engine problem depending on results.

So this afternoon l got my tools out and pulled both carbs. Took the opportunity to give them a quick blast with carb cleaner, not that they were dirty. Upon doing this l noticed the length of the air bleed screws protruding from the locknuts was different from carb to carb. I cracked open the lock nuts on one carb and found that one screw was 2 turns out and the other on the same carb was 1 1/2 turns. Proceeding onto the other carb, both locknuts cracked, l found that both air bleed screws were in the fully closed position.  Interesting...as l never touched them...

So decided not to swap carbs from one side to the other at this moment, instead l adjusted all the air bleed screws so that they were all 1 1/2 turns out as a starting point. I've now put the carbs back as they were without any linkage connected and got the engine started and warmed up. Running lumpy but once warm and l re synced the carbs at idle again things smoothed out as before. now for the part throttle bit. Decided to sacrifice a feeler gauge to be 100% cut in half trapped between the idle screw and throttle stop arm as before to pick up and hold the revs. 

Surprisingly both carbs with the Snail read the same reading of 13 on the Snail. Pulled  the feeler gauges out and they returned to idle reading the same 5.5 in the Snail. It's a great feeling making progress and finally getting to the bottom of something.

Just need to take it for a run and see how it goes. Not sure how much the air bleed screws should be turned out or in. Alot have mentioned about flat spots if out or in to much so trail and error guess at this stage.

 

Richard, close the air bleeds. Listen to Stan, he knows a thing or two. They are there merely to balance a low cylinder. This is maybe caused by a slightly bent or misaligned throttle shaft, maybe a throttle butterfly isn't 100% centered. Whatever, you bring the low cylinder up with the bypass screw. Once off idle, the bypass ceases to work. FYI, mine has one low and is balanced with the screw, the rest are closed.

Please, throw away the washers and the cut feeler gauge. Useless.

You need the airflow synchronized when you are driving, when you are pulling on the linkage with your right foot. PERIOD! Any other method is a fools game.

I am glad you figured out the difference L/R and didn't resort to swapping sides. But that is a good method for troubleshooting.

I'm with Stan.  I've always started with the air bleeds fully closed and then open one on a low (front to back) cylinder just to balance that side.  Never heard the folklore about air bleeds causing a flat spot.....That is usually caused by something entirely different.

I have also made a manual throttle similar to Danny's, but mine is a rod attached to one of the case studs near the distributor.  The rod has a turnbuckle on the other end and a way to clamp onto the throttle cable arm on my (gasp!) heim-jointed, hex bar linkage.  Once assembled, I can dial in whatever engine speed I want by turning the turnbuckle to get me to 1,500 - 3,000 rpm to make sure things are OK on the mains.  I always disconnect the linkage at idle to sync things there.  After that, I use the same process as Danny and Stan.  

Looks like you found the culprit in that opened air bleed.  Those little buggers can make a big difference.  That's probably why people recommend to close them unless you need to balance both throats on the same carb - that's what they're there for, but 12 to 18 is a big difference on one carb.  As mentioned, I would turn them all in closed, then only open one on a low throat on the same side carb to bring it up to sync on that side.

Come to think of it, maybe you can use this as a great educational opportunity:

You now seem to have it running to your satisfaction at idle and (hopefully) at 2,500 with the washers on the idle stops, which, BTW, I'm OK with.  Take it out and drive it and see how it performs.

Then, try running all of the bleed screws out 1 turn and re-sync everything at idle and at 2,500 rpm.  I would be curious to see what effect you see in twiddling the bleed screws when running at 2,500 - do they only affect the low idle or do they have significant effect at 2,500 rpm?  Please let us know the result if you do this.

Once sync'd with the bleed screws somewhere around 1 turn each, take it out and drive it and see how it performs and report back on any differences, positive or negative, that you feel.  

Reading Tomlinson's book on carb "theory" is one thing, first-hand experience is a whole 'nuther thing.

Don't make me go out into the garage and start twiddling with Pearl's bleed screws just to see what happens.  I am known to really screw things up once given a screwdriver.......

Thanks,  Gordon

Gordon, as I stated above, the idle air bypasses do nothing once the throttle butterflies are opened. The bypass "may" very slightly lean the just off idle speed mixture, but that's about it, I'd expect. BTW, we are talking Webers here and not Dells.

You can be OK with the washers on the idle stops, but you have to concede it means absolutely nothing in the grand scheme of things. The entire linkage MUST be pulled from where the loud pedal pulls it from to actually be able to set high rpm synch. Really.

Gordon Nichols posted:

Come to think of it, maybe you can use this as a great educational opportunity:

You now seem to have it running to your satisfaction at idle and (hopefully) at 2,500 with the washers on the idle stops, which, BTW, I'm OK with.  Take it out and drive it and see how it performs.

Then, try running all of the bleed screws out 1 turn and re-sync everything at idle and at 2,500 rpm.  I would be curious to see what effect you see in twiddling the bleed screws when running at 2,500 - do they only affect the low idle or do they have significant effect at 2,500 rpm?  Please let us know the result if you do this.

Once sync'd with the bleed screws somewhere around 1 turn each, take it out and drive it and see how it performs and report back on any differences, positive or negative, that you feel.  

Reading Tomlinson's book on carb "theory" is one thing, first-hand experience is a whole 'nuther thing.

Don't make me go out into the garage and start twiddling with Pearl's bleed screws just to see what happens.  I am known to really screw things up once given a screwdriver.......

Thanks,  Gordon

It's much easier EXPERIMENTING with the people's carbs isn't it Gordon?

"It's much easier EXPERIMENTING with other people's carbs isn't it Gordon?"

Yeah, I thought about that an hour after I posted, but I realized two things:

1.  Whatever Rich does with the air bypass screws, he'll learn something of value, it won't hurt anything, it may improve something and it is totally re-settable in under five minutes, plus: 

2.  It's now made me curious and I'll be out there, today, playing with mine on my Dells just to see what the heck they affect.  Not that I don't believe Dan, I do...  But self-experienced knowledge is a wonderful thing and I can add that experience to expand my service manual for my son, for when he inherits my cantankerous old car.  With all of the custom stuff on it, even I need a comprehensive service manual (my tired old brain ain't gettin' any younger!).

Gordon, I hear you. I made a 3 ring binder and put my Vintage Spyder manual, my Raby engine book, the Webasto book, and a couple hand-written schematics. I keep it behind the seat. That way my poor memory doesn't need to be relied upon!

Stan, yes, I'm aware. So then you only need one washer then on the 3/4 side, and you can sync the 1/2 carb with the cable barrel, eh? That's what we did on Lenny's except we used the gas pedal. The point I was making is that two separate washers, one on each carb stop, does jack squat, no matter what linkage you own.

No, if you disconnect the linkage and put a similarly-thick washer under each idle stop, it should open each throat the same without using the linkage.

Unless I'm mistaken (now THAT might be a first    ) that is what Rich was doing.

On the Service manual, I kind-of went overboard and scanned in my old, pertinent reference stuff and then organized everything like a Bentleys, but I added a host of hyper-links to help navigate around.  Now, I bring up the table of contents and just click on the stuff I want to see and it goes there.  Wrote the whole thing in Word.  I've attached the chapter on the Electrical system, if you're curious - It's the most involved with lots of scans.  Just go to the reference at the top and click on what you want to see.

 

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Gordon just for curiosity I'll give it ago. Got a road trip this Saturday so will have ago.

The air bleed screws are still set a 1 1/2 turns out. Will start the road trip this way and turn them in all the way in has suggested  and re-sync them and see if there is any difference.

Im agreeing with Danny that they only come into play when the butterflies are closed, but still curious.

What l do find annoying, when it went on the rollers, the guys playing with the jets etc never picked up the carbs were out of Sync of idle. I knew something wasn't quite right but in away accepted it.

What I'm thinking is, if at idle the carbs read 5 on the Snails @ 850rpm and at 2500rpm they read 18 and 12. Then what what would they have read at 7000rpm.??  Not that I'm going to rev the **** out of it to see. Do you see what I'm getting at?? At idle if one carbs reads 5 and the other reads 3.5, that makes a big difference how smooth it runs. So with a difference of 6 at 2500rpm. It should be a considerable difference in how much smoother and snappier it delivers it power. Would you guys agree?? Or am l thinking to much lol. Easily done.

Gordon Nichols posted:

No, if you disconnect the linkage and put a similarly-thick washer under each idle stop, it should open each throat the same without using the linkage.

Unless I'm mistaken (now THAT might be a first    ) that is what Rich was doing.

 

Yes, I know exactly what Richard was doing. I guess nobody gets what I'm saying. Who gives a flying whatever about behavior with washers? No linkage, idle stops, ok. that's necessary. But I only care what the synch is when the LINKAGE is pulling on it. Because driving conditions matters. Get it? Jeez, I hope so. You people(Gordon) are dense. The question is why would you want to do that and what does it accomplish?I'm out.

Honestly, Richard, if you are at 7K you are at WOT, and they will read the same, if they could be read. The only time synch really matters is at idle and small openings such as cruise.

Danny l think we all get what your saying. I always synced the carbs at idle without any linkages/cables. Once done l know if l connect the cables/linkages and if things change l know its down to the latter. I'll re check with the Snail once everything is hooked up to just to be sure.

Might be the Long way round But l guess we take the same journey just  differant routes but we all get to the final destination.

There was mention on here about hex bar linkage being good but no comments on what makes it good. Here's a few photos of why I think they are junk. The first photo shows the throttle levers installed the normal way on the hex bar. The next shows them reversed so I could check to see if they are drilled perfectly on center to insure their pull/push radius is the same. The third photo shows that they are not. The holes are off by almost 1/8". This means that they will never pull (or push) equally at the carburetor throttle plates making carb. sync. impossible at any position. Just to be sure about this, I installed appropriate pins in both ends of the levers and measured the inside distance with a micrometer (Helios caliper) to see what the difference was.     .104" was the difference. The fourth photo shows it gets worse. When you tighten up the two set screws on each lever they move even more out of alignment and each time you loosen and re-tighten them,  the alignment changes to something else. About the time you think you got them pretty close to sync, the throttle stop screws aren't touching the stops on one of the carbs. Lastly, IF you get everything in sync, go stomp on the pedal a few times while taking a cruise, they will not be in sync by the end of the day. The set screws get just enough torsion on them to dig in and change the lever position slightly but enough to create that crappy "out-of-sync" performance at cruising speeds

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So just for Gordon and anyone else's curiosity during my road trip l screwed the air bleed screws in. I did have to screw the idle screws in alittle as the idle dropped whilst screwing the air bleed screws in and then had to re-sync the carbs and check the linkages were opening at the same time also. 

Upon driving it, if honest l felt no difference what so ever. It didn't feel snappier or anything, so in my circumstances it must be minimal whether the air bleed screws are in or out. 

 

I kinda thought that's what would happen - nothing.  I was mostly curious about the 1-1/2 turns they used to be out.....  That's a good bit, but as you found, once everything is balanced again at where-ever you start with, turn-wise, it's in sync at idle.

All the air bleeds do is allow a little more air into the mix for that one throat when the throttle plates are at or close to idle.  You saw that when they effected the idle speed.  They pretty much lose any effectiveness off-idle as the throttle plate edge approaches the transition ports and the bleed becomes less active.  At least that's how I read about it.

Thanks for taking the time to play with them and report back!

I've got a minute, so I'd like to elaborate on why I hate the hex-bar, and what I did in pursuit of making it hex-bar work before I just went to sync-link.

The first problem, the one everybody thinks is the only problem, is the ball-joint and spring set-up on the ends. The heim-joint/bearing fix is touted as being the do-all/end-all, but all it does is fix one of four actual issues.

The second problem is with the heim-joints on the drop-arms. At best, they are exposed to crud and dirt, and require lubrication to stay limber. At worst, they are tight, and sticky. I've had more bad joints that good-- I have at least 20 rods of various length with "good" and "no good" ends on them. The "good" ones are pretty iffy. 80+% of them are "no good"

The third and fourth problems are with the actuator arms. The hex cuts in the arms are not always oriented the same, arm to arm. If they aren't, you're screwed-- the geometry side to side won't and can't be identical. It's good to have a stack of them, so you can flip them backward and see how they meet up in the middle. If you get two arms that are the same and again, I've got a stack of about 10-- you still will have holes for the drop-link heims that aren't lined up. I would get a matching set of arms, fill both holes with JB Weld, and re-drill both of them to get the geometry identical.

After all that- picking through a stack of parts to get arms that match, heims that are limber, drilling new holes in the arms, and doing the heim-joint modification on the supports for the hex, I had something that worked. For about a year.

Eventually the axles on the heim-joint modifications (on the ends) wear down and give you slop. Invariably, one drop-link heim would lock up at the worst possible moment. In the end, I just lost the will to make something so incredibly cobbled together work right.

Lots of guys have them, can't make them work, then blame the carbs. I've often said that 90% of carb problems are ignition. Of the remaining 10%, at least half are linkage. You really don't have to live with that.

Forewarned is forearmed.

Stan, now you've done it...I had a hex-bar set-up I was going to list here, now no one will buy it.

On another note, I love, love, love my Vintage Speed (Taiwan VW speed parts company) linkage. I believe (Left-Coast) Air-cooled Bruce is running and loves his too.

It's a center-pull but with slip-ends at each throttle arm.

I set mine up after syncing so each end has .001 clearance. As the engine increases in width from heat, the clearance increases but they are still perfectly in sync. at any rpm. I think it's a system that's pretty close to perfect, at least for a town the size of mine.

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